I hate, in fact loath when I'm told that there will be a roaming mic for speeches.
It's bad enough that sometimes people will either face the bride & groom - therefore their back will be towards me, face the guests...but worst still, walk around the room as if they have ants in their pants.

I encourage wherever possible, the use of a lecturn and can deal with this - but a roaming mic throws all sorts of issues.

Focusing on the audio elements, what are your best approaches of getting good audio if the venue won't allow you to tap into their sytem?

I've taped my lapel mic on to the microphone, but have to admit that it's ugly.

The only drawback I've encountered is that the reception staff aren't very excited about the prospect of someone taping a recorder and mic to their mic. As long as the on/off buttons on the house mic are exposed there's never been any backlash. And I learned the hard way to also mic the DJ/Band mic if the toasts are supposed to happen on the house mic and vice versa. Inevitably the other mic will be used for something that, when you're editing, you wish you had a better version of.

If the toast mic is wired I also split off the signal from the toast mic to a wireless transmitter that sends its signal to a second camera.

XLR pass-through and adjustable outputs to send a signal to your wireless transmitter.

I've given up trying to patch into the DJ's board. More often than not if you ask the DJ what type of cable they'd need to send a line level signal to a wireless receiver or recorder they give you a blank stare. Unlike the people that run the mixing boards in churches the average DJ doesn't know much about the equipment they're using. (Unless its the owner of the DJ company, in which case they'll go out of their way to get you set up.)

for me, I use my standard reception audio setup. AKG Bluline Hypercaroid on the camera and my trusty Sennheiser E604 drum mic with my wireless plugun transmitter on a stand in front on one of the DJs speakers. That way no matter wheter they stand still or run a marathon around the room, I get audio coverage.
Of course there is always the threat of great physical harm to them if they don't use the podium

We still use the irivers but have not taped it onto the mic - good suggestion. I've been trying to find a solution that will allow us to have a direct feed to the camera so that I don't have to sync in post. In saying this, the trusty iriver has served us well as is relatively small.

for me, I use my standard reception audio setup. AKG Bluline Hypercaroid on the camera and my trusty Sennheiser E604 drum mic with my wireless plugun transmitter on a stand in front on one of the DJs speakers. That way no matter wheter they stand still or run a marathon around the room, I get audio coverage.
Of course there is always the threat of great physical harm to them if they don't use the podium

O|O
\__/

This is a great tip - not plugged our drum mic into the wireless before = always have had it in an iriver.

Biggest issue is when the reception venue has house speakers that are way higher than our stand...our car is like a truck load of equipment and generally we try to just take our smaller light stand rather than our bigger one out.

Yeah, house systems are terrible. Small speakers way up high. YUCK!
Honestly, while it might cost a bit of money I would recommend getting rid of the shotgun mic and getting a hypercaroid. Much better response indoors than a shotgun. Howerver you could still run a mic back to your wireless even with the high ceilings. Mount a lav to your stand (the drum mic doesn't have the high end needed for people speaking) and just get it as high as possible, as close to the ceiling as possible. If you can get within about 3 feet you should be able to get enough decent sound to make it workable. Of course that depends on the lav mic you're using. Methinks a hypercaroid is the best answer.